Samsung Galaxy NX has a
20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features DRIMe IV processor. On the other hand, Pentax K-3 has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Prime III processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-3's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the Galaxy NX and K-3 sensor size comparison.
Samsung Galaxy NX and Pentax K-3 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.